10th International symposium on Negative Ions, Beams and Sources - NIBS'26

Phase Mixing and Compensation Transitions in Single and Multiple Negative-Ion Beams

Sep 21, 2026, 5:00 PM

2h

Hume Hotel

Poster Poster Session 1

Speaker

Dr Chen Zuo (Huazhong University of Science and Technology)

Description

Space charge compensation (SCC) is a key process in negative-ion beam transport, because ionization of the residual gas generates secondary charges that can partially or even overcompensate the beam space charge and thereby modify beam transport. Previous studies have shown that the compensation state depends on background-gas conditions, secondary-particle production and loss, and, in multibeam systems, electrostatic coupling among neighboring beamlets. Recent work has further indicated that, in overcompensated regimes, secondary electrons can accumulate and reduce the degree of overcompensation. Motivated by these issues, we formulate a reduced one-dimensional hybrid PIC-MCC model in which one or three prescribed slab-like negative-ion beams are embedded in the transverse coordinate, while beam-born secondary positive ions are treated kinetically.
The aim of the model is not to construct a full beamline predictor, but to isolate the trapping, oscillation, and collisionless phase-mixing dynamics of secondary ions in beam-induced potential wells, and to examine how these ion dynamics interact with electron effects in overcompensated regimes. Within this framework, we test whether phase mixing supports a useful coarse-grained description of the trapped secondary-ion population. In particular, rather than interpreting the redistribution as true thermodynamic thermalization, we introduce an effective ion temperature based on the second moment of the coarse-grained transverse velocity distribution and use it as a reduced descriptor of secondary-ion confinement and escape. This provides a compact way to analyze SCC buildup in the single-beam case. On this basis, we then examine how electron accumulation in overcompensated regions modifies or limits overcompensation, and how electrostatic coupling changes these compensation transitions in the three-beam case. The model is therefore intended to provide a compact, physics-oriented framework for comparing single- and multibeam compensation dynamics, with particular emphasis on the respective roles of trapped ions and accumulated electrons in the onset and limitation of overcompensation.